In recent years, the performance of nonaqueous electrolyte secondary batteries has been improved. Consequently, high-output power supplies for a portable electronic appliance, a hybrid car, and an electrical car, and for electric power storage, have been realized. Thus, the development of techniques of reducing the resistance of an assembled battery having a large number of series-connected battery cells is a pressing problem. Generally, high-output secondary batteries capable of supplying a large current generate a large amount of heat at high-speed charging/discharging. Accordingly, the generation of heat can be suppressed by reducing the internal resistances of the assembled batteries. Consequently, the cooling of the batteries is facilitated. Thus, an increase in the lifetime of assembled batteries can be expected by reducing the generation of heat. Also, an increase in the energy density of assembled batteries can be expected by reducing the resistance thereof.
Meanwhile, a flat cell is an example of a battery configured by using an armoring member laminated with a resin layer, a metal layer, and a resin layer. This battery has a simple structure in which flat electrode tabs, for example, formed in strip-like shape are drawn out of the armoring member. Usually, when an assembled battery having flat cells is formed, these electrode tabs are connected to each other. Currently, various methods, such as a bolting method, a resistance welding method, a soldering method, a laser welding method, and an ultrasonic welding method (see, e.g., JP-A-2006-210312), are employed as the techniques of connecting electrode tabs to each other.
In current most common flat cells, a positive electrode tab is formed of aluminum. A negative electrode tab is formed of nickel. Therefore, in the case of manufacturing an assembled battery having these flat cells connected in series, the aluminum tab and the nickel tab are connected to each other.
Meanwhile, a negative electrode active substance of such a flat cell has an electric potential which is equal to or higher than that of a conventional negative electrode active substance. Accordingly, a flat cell using aluminum tabs as both the positive and negative electrode tabs is studied (see, e.g., JP-A-2006-92973).
In the case of manufacturing an assembled battery having series-connected flat cells, which use aluminum tabs as both positive and negative electrode tabs, the aluminum tabs are connected to each other. Generally, the electrode tab formed of aluminum has an advantage that corrosion of the tab due to electrolyte is hard to occur, whereas the electrode tab formed of aluminum has a disadvantage that the formation of an oxide film on a surface of the tab results in increase in the resistance thereof. Accordingly, the oxide film largely affects the connection between the aluminum electrode tabs. The resistance of the connection therebetween is higher than that of the connection between the aluminum tab and the nickel tab. More particularly, in the case of manufacturing a large battery which need to series-connect a large number of battery-cells, the contact resistances respectively corresponding to the connections of adjacent ones of the series-connected battery-cells are added and increase according to the number of the cells. Consequently, the oxide film greatly affects the resistance of the assembled battery.
Meanwhile, a method of connecting aluminum tabs to each other by ultrasonic welding is thought of as a most suitable method for connecting the aluminum tabs to each other. This is for the following reasons. That is, the tabs can be connected by welding, so that the resistance of the assembled battery is less affected by an increase in the resistance, which is caused due to the oxide film, and that battery materials are not heated and are less damaged. However, the ultrasonic welding is performed according to the principle that connection surfaces are connected to each other by being resonated. Therefore, it is difficult to connect surfaces having large areas to each other at a time. When the connection of the surfaces is performed by a commonly-used ultrasonic welder, the electrode tabs are connected to each other in a spot-like manner, that is, the electrode tabs are partly connected to each other. Thus, for example, only parts of the tabs, which are other than the surfaces thereof welded to each other by ultrasonic-welding, are put into contact with each other. That is, the contact between the tabs is insufficient. Additionally, an increase in the contact resistance between the tabs and the resistance of a connection portion between the electrode tabs are likely to be nonuniform. Further, in a case where the body of the battery is shaken for a long time so as to achieve sufficient contact therebetween, a burden is imposed on the connection portion between the electrode tabs. Consequently, a crack or a rupture has become increasingly likely to occur at the connection portion.
Therefore, in the case of the aluminum electrode tabs which are manufactured using a connection method such as the ultrasonic welding method, and which are partly welded to each other, it is difficult to reduce the resistance between the electrode tabs.